DEVELOPMENT OF A SEDIMENT REDOX POTENTIAL MODEL FOR THE ASSESSMENT OFPOSTDEPOSITIONAL METAL MOBILITY

A numerical model was developed to simulate the vertical profile of th e redox potential in benthic sediments. The benthic sediments were sub divided vertically into six zones, each with different microbial and c hemical reactions: aerobic respiration, denitrification, manganese red uction, iron reduction, sulfate reduction, and methanogenesis. Microbi al degradation of organic matter and subsequent chemical reactions of interest were formulated using stoichiometric relationships and consid ering the vertical advective/dispersive transport in the sediments. Th e kinetics of utilization of the different electron accepters during t he biodegradation of the organic matter were described by a Monod-type formulation. Eleven coupled differential equations were derived and s olved interactively utilizing an iterative multistep numerical method. The model input parameters include the rate of solid deposition, conc entrations of electron accepters in the water overlaying the sediments , activities of the benthic fauna, and molecular diffusion. The model simulates the redox potential as well as eleven chemical constituents in the sediments, three solids (particulate organic matter, manganese oxide, and iron oxide), and eight dissolved species (oxygen, nitrate, sulfate, ammonia, dissolved manganese, dissolved iron, sulfide, and me thane). The model demonstrated that accurate estimates of the flux of primary electron accepters and donors from the overlying water to the benthic sediments is important to determine the redox conditions in se diments. Bioturbation and the rate of pore-water infiltration are proc esses that have a major influence on this flux.